thermal: generic-adc: Add ADC based thermal sensor driver

In some of platform, thermal sensors like NCT thermistors are
connected to the one of ADC channel. The temperature is read by
reading the voltage across the sensor resistance via ADC. Lookup
table for ADC read value to temperature is referred to get
temperature. ADC is read via IIO framework.

Add support for thermal sensor driver which read the voltage across
sensor resistance from ADC through IIO framework.

Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>